In a wireless network having a resource restricted device, maintaining such resource restricted devices commissioned into a network is an issue. Indeed, such resource restricted devices are often in a not listening state, during which no messages, including configuration messages, e.g. indicating a change of operating channel, can be received.
A resource restricted device in the sense of this patent application is a node which is operating with very low power and may even have no power storage. It may be for example a ZigBee Green Power Device (also referred to by GPD). This kind of device may be able to transmit or receive only at some opportunities, for example after harvesting energy from the environment (e.g. in case of a light detector using the solar power to operate) or from the actuation by a user (e.g. in case of a battery less switch). Such reception opportunities can thus not always be scheduled in advance.
The bidirectional communication included in Green Power specification solves partially these issues. In accordance with this specification, some nodes in the vicinity of a resource restricted device may take the role of a proxy, which gathers all the messages addressed to the resource restricted device, and forwards it to the resource restricted device only when a reception opportunity is offered. As there may be a plurality of nodes that can take the role of proxy around a single resource restricted device, one of them can be elected as a master node (also referred as a TempMaster), so that only one node is transmitting a message at a time to the resource restricted device. This is crucial in Green Power specification to maximize the reception probability of the resource restricted device; because the reception opportunity of the resource restricted Green Power Device is—due to its power limitations—strictly time limited, so that the devices taking the role of the proxy cannot backoff or retry their transmission in cast of collision. The transmission via TempMaster is used for example, when responding to a GPD packet. For this, a sink node, which is logically bound with the resource restricted device, selects a TempMaster from among the nodes in the vicinity of the resource restricted device (the devices may be proxies and/or sinks capable of Sink Table-based forwarding, including the sink node itself, if in range). The TempMaster shall forward Green Power Device Frames (GPDF) from the sink node to the resource restricted device. When the sink node has to transmit a message to the resource restricted device, it forwards the message to the TempMaster, which awaits the next reception opportunity of the resource restricted device to deliver the message.
When a ZigBee Green Power Device is transmitting a Green Power Device Frame, this GPDF may include a flag RxAfterTx=true, which indicates that after the transmission of this message a reception window will be opened. During this reception window, the TempMaster delivers the message to the GPD, (at least) 5 ms after the reception of the GPDF with RxAfterTx=true. Thus, there is a delay between the message generation and message delivery.
This mechanism of ‘Reception After Transmission’ was meant to be used for infrequent events, like channel or key update. Indeed, assuming very tight energy budget for resource restricted device, it is better to spend the energy on several attempts of transmission (the same message may be repeated in sequence 2 or 3 times to maximise the probability of reception) in broadcast mode (with a special address or no address i.e. to multiple potential receivers), rather than on sending in unicast to a particular receiver and waiting for an acknowledgement. In most wireless systems, listening has the same, or even higher, cost as/than transmitting. Moreover, the system may have no energy to act upon lack of acknowledgement at the timeout.
This transmit only behaviour with retries is fine if the GPDF non-reception on the GP infrastructure device (i.e. proxy and/or sink) side is caused by a temporary problem, like interference, fading or portability/proxy switching (which will be resolved by the GP protocol). However, if the network parameters (e.g. key, channel) have changed when the GPD was not operational, it will have no means of discovering the problem. Especially in the case of channel change, the network will likely also have no means of discovering the problem, since—having moved to the new channel—it will no longer receive the GPD's messages on the old channel. This problem is even more acute if the resource restricted device is communicating aperiodically (as may e.g. a user operated device, e.g. a light switch, or a device which transmission intervals depend on the amount of energy available (harvested), e.g. a sensor powered by a small solar cell, or flow of air or fluid): the system does not know when to expect a message and when to assume the resource restricted device is lost.